The present invention relates to an oil-impregnated power cable. More particularly, it is concerned with a power cable impregnated with an oil which oil is obtained from starting materials of a specific composition and by a specific manufacturing process, and having an insulation layer which is formed of a composite film of polyolefin film and insulating paper.
In recent years, the demand for high load voltages an oil-impregnated power cables has been increasing. And to meet this demand there have been made various improvements with respect to the structure of power cables, particularly the insulation layer thereof. For example, an attempt has been made to substitute the conventional insulating papers by a polyolefin film having a higher dielectric strength and a smaller dielectric loss for use in the cable insulation layer. However, polyolefin films impregnated with conventional impregnating oils sometimes become swollen, which causes drawbacks, e.g. increased oil flow resistance.
In order to avoid such drawbacks associated with the prior art, it has been proposed to use both a polyolefin film and an insulating paper and thereby form an insulation layer for power cables. In this case, however, if such a method as has been applied for the manufacture of oil-impregnated condensers, namely a mere overlapping and winding of polyolefin film and insulating paper, is applied to the manufacture of cables, there sometimes occur shears and wrinkles between insulation elements because of a cable bending.
It is therefore necessary to effect bonding between polyolefin film and insulating paper by some means so as not to cause shears and wrinkles. In this case, care must be exercised so as not to lower the adhesive strength against a long-term impregnation in oil. Conventional impregnating oils still leave something to be desired in these respects. Of course, even with a power cable using such composite film of polyolefin film and insulating paper, the foregoing problem of oil flow resistance is an important subject.
It is an object of the present invention to provide an oil-impregnated power cable free from the aforesaid drawbacks encountered in the conventional oil-impregnated power cables.
It is another object of the present invention to provide an oil-impregnated power cable permitting application of increased load voltages, having smaller dielectric loss and oil flow resistance and undergoing neither shears nor wrinkles caused by a cable bending.
Other objects of the present invention will become clear from the following description.